The catalyst systems composed of ZnBr2 and different phosphonium salts were examined for solvent-free synthesis of cyclic carbonates from CO2 and terminal epoxides under mild conditions. Among the catalysts investigated, ZnBr2-Ph4PI was found to be the best while those of ZnBr2-phosphine oxide (Bu3PO or Ph3PO) show no catalytic effect. It is apparent that the halide ions of phosphonium salts have an essential role to play in the reaction. The catalytic activity of ZnBr2-Ph4PI increases with a rise of Ph4PI to ZnBr2 molar ratio up to 6, above which there is little change in catalytic activity. We observed that with a rise in ZnBr2 to Ph4PI molar ratio, there is increase in epoxide conversion but decline in TOFPO (estimated based on the site number of Zn2+). The effect of water on the reaction was investigated for the first time. We found that the presence of even a trace amount of water would result in a marked decline in reactivity, and the observation provides a valid explanation for why reproducibility of results is poor among researchers so far. The influences of other parameters such as reaction temperature and CO2 pressure on the catalytic performance of ZnBr2-PPh4I were also studied. It is shown that the catalyst is sensitive to reaction temperature, and a rise of reaction temperature up to 130 °C favors the formation of cyclic carbonates. We observed that activity increases with rise in CO2 pressure and reaches a maximum at an initial CO2 pressure of 2.5 MPa. Moreover, a plausible reaction mechanism has been proposed.
Scopus Subject Areas
- Process Chemistry and Technology
- Carbon dioxide
- Cyclic carbonate
- Terminal epoxide
- Tetraphenylphosphonium iodide
- Zinc bromide